Two US fighter jets have dropped four unarmed bombs in Australia’s World Heritage-listed Great Barrier Reef marine park after a training exercise went wrong.

The AV-8B Harrier jets took off from the aircraft carrier USS Bonhomme Richard and each released an inert bomb and an unarmed explosive bomb off the coast of Queensland, the US 7th Fleet said in a statement on Saturday.

The four bombs were dropped in more than 50 meters (164 feet) of water away from coral that was supposed to have minimized possible damage to the reef, the statement said. None of the bombs exploded.

The training mission went gone wrong as the jets from the 31st Marine Expeditionary Unit were originally told to drop the bombs at the Townshend Island bombing range. That plan was aborted after the controllers said the area was not clear of hazards.

The pilots then went ahead with an emergency jettison as they were not allowed to land with bombs on board and both were low on fuel, the navy said.

The US Navy and Australian authorities are now investigating the incident.

Coral on Australia’s Great Barrier Reef.(Reuters)

The failed training mission happened on the second day of the biennial joint training exercise Talisman Saber that brings together 28,000 US and Australian military personnel over three weeks.

Environmentalists and anti-war activists Graeme Dunstan, who is against the joint exercise, said that the incident proves the US cannot be trusted to protect the environment.

“How can they protect the environment and bomb the reef at the same time? Get real,” Dunstan told AP.

The Great Barrier Reef is a UNESCO World Heritage Site and is the world’s largest coral reef system, made up of over 2,900 individual reefs and 900 islands, stretching for more than 2,600 kilometers (1,600 miles) along the Australian northeast coast.